This invention relates to electrophotographic toners and in particular to toners for utilization in flash or radiant fusing which are heat and light sensitive.
The formation and development of images on the surface of photoconductive materials by electrostatic means is well known. The basic electrophotographic process, as taught by C. F. Carlson in U.S. Pat. No. 2,297,691, involves placing a uniform electrostatic charge on a photoconductive insulating layer, exposing the layer to a light-and-shadow image to dissipate the charge on the areas of the layer exposed to the light and developing the resulting electrostatic latent image by depositing on the image a finely-divided electroscopic material referred to in the art as "toner". The toner will normally be attracted to those areas of the layer which will retain a charge, thereby forming a toner image corresponding to the electrostatic latent image. This powder image may be transferred to a support surface such as paper. The transferred image may subsequently be permanently affixed to the support surface as by heat. Instead of latent image formation by uniformly charging the photoconductive layer and then exposing the layer to a light-and-shadow image, one may form the latent image by directly charging the layer in image configuration. The powder image may be fixed to the photoconductive layer if elimination of the powder image transfer step is desired. Other suitable fixing means such as solvent or overcoating treatment may be substituted for the foregoing heat fixing steps.
Several methods are known for applying the electroscopic particles to the electrostatic latent image to be developed. One development method, as disclosed by E. N. Wise in U.S. Pat. No. 2,618,552 is known as cascade development. Another method of developing electrostatic latent images is the "magnetic brush" process as disclosed, for example, in U.S. Pat. Nos. 2,874,063; 3,103,445; 3,251,706 and 3,357,402. In this method, a developer material containing toner and magnetic carrier particles is carried by a magnet. The magnetic field of the magnet causes alignment of the magnetic carrier into a brush-like configuration. This "magnetic brush" is engaged with the electrostatic latent image-bearing surface and the toner particles are drawn from the brush to the electrostatic latent image by electrostatic attraction. Other methods of development include "powder cloud" development as disclosed, for example, by C. F. Carlson in U.S. Pat. No. 2,221,776; "touchdown" development as disclosed by R. W. Gundlach in U.S. Pat. Nos. 3,166,432 and 3,245,823 by Mayo; and "Cascade" development described in U.S. Pat. No. 3,099,943.
Although all of the above mentioned developing techniques and others are presently used almost exclusively for black and white reproduction, they are capable of forming images in other colors and combinations of colors. As in other color systems, electrostatographic color systems are generally based on trichromatic color synthesis of either the additive or substractive color formation types. Thus, where electrostatographic systems are operated in full color, toner or developing particles of at least three different colors must be employed to synthesize any other desired color. As a rule, at least three-color separation images are formed and combined in register with each other to form a colored reproduction of a full colored original. In color electrophotography, as described, for example, in U.S. Pat. No. 2,962,374 to Dessauer, at least three electrostatic latent images are formed by exposing an electrostatographic plate to different optical color separation images. Each of these electrostatic latent images is developed with a different colored toner, after which the three-toner images are combined to form the final image. This combination of the three-color toner images is generally made on a copy sheet, such as paper, to which the toner images are permanently affixed. The most common technique for fixing these toner images to the paper copy sheet is by employing a resin toner which includes a colorant and heat fusing the toner images to this copy sheet. Images may be fixed by other techniques such as, for example, subjecting them to a solvent vapor. Color "highlight" systems wherein a copier may contain black and one or two highlight colors are also known. Such a copier can produce either black copies, single color copies of another color or black copies with color highlighted areas.
Many forms of image fixing techniques are known in the prior art, the most prevalent of which are vapor fixing, heat fixing, pressure fixing or a combination thereof. Each of these techniques, by itself or in combination, suffer from deficiencies which make their use impractical or difficult for specific xerographic applications. In general it has been difficult to construct an entirely satisfactory heat fuser such as a roll fuser, having a short warm up time, high efficiency, and ease of control. A further problem associated with heat fusers, especially radiant fusers, has been their tendency to burn or scorch the support material. Pressure fixing methods, whether heated or cold, have created problems with image offsetting, resolution degradation and producing consistently a good class of fix. On the other hand, vapor fixing which typically employs a solvent has proven commercially unfeasible because of the odor and solvent recovery problems involved. Equipment to sufficiently isolate the fuser from the surrounding ambient air must by its very nature be complex and costly.
With the advent of new materials and new xerographic processing techniques, it is now feasible to construct automatic xerographic reproducing apparatus capable of producing copy at an extremely rapid rate. Radiant flash fusing is one practical method of image fixing that will lend itself readily to use in a high speed automatic process. The main advantage of the flash fuser over the other known methods is that the energy, which is propagated in the form of electromagnetic waves is instantaneously available and requires no intervening medium for its propagation. As can be seen, such apparatus does not require long warm up periods nor does the energy have to be transferred through a relatively slow conductive or convective heat transfer mechanism.
Although the flash fusing systems such as disclosed in U.S. Pat. No. 3,903,394; U.S. Pat. Nos. 3,474,223 and 3,529,129 of the prior art perform satisfactorily with the black toner of conventional copying processes, the flash fusing is not as efficient when fusing colored toners. Colored toners absorb much less light and therefore greater energy input to fuse than black toners. Further, when copying a full color image which contains portions which are black and portions of low light absorbing colors such as yellow the flash fusing is difficult as there is a tendency either to overfuse the black portions in order to fuse the yellow or conversely to not effectively fuse the yellow although the black and darker colors are properly fused. Proper flash fusing of images of mixed colors requires a narrow range of energy input to achieve complete fusing. It is difficult to maintain a commercial copier narrow parameters such as a narrow range of flash fusing energy input for a long period of time. Therefore as can be seen it would be desirable if it was possible to fuse full color images which would absorb substantially equally the output of a flash fuser. Further there is a need for a method of flash fusing colored toner images such that uneven fusing does not give an irregular surface appearance.